The use of stents in various surgical, interventional cardiology, and radiology procedures has quickly become accepted medical practice as experience with stent devices accumulates and as the advantages of stents become more widely recognized. Stents are often used in body lumens to maintain open passageways such as the prostatic urethra, the esophagus, the biliary tract, intestines, and various coronary arteries and veins, as well as more remote cardiovascular vessels such as the femoral artery.
Stents are often used to treat atherosclerosis, a disease in which vascular lesions or plaques consisting of cholesterol crystals, necrotic cells, lipid pools, excess fiber elements and calcium deposits accumulate in the walls of an individual's arteries. One of the most successful procedures for treating atherosclerosis is to insert a deflated balloon within the lumen, adjacent the site of the plaque or atherosclerotic lesion. The balloon is then inflated to put pressure on and “crack” the plaque. This procedure increases the cross-sectional area of the lumen of the artery. Unfortunately, the pressure exerted also traumatizes the artery, and in 30-40% of the cases, the vessel either gradually renarrows or recloses at the locus of the original stenotic lesion. This renarrowing is known as restenosis.
A common approach to prevent restenosis is to deploy a metallic stent to the site of the stenotic lesion. Although metallic stents have the mechanical strength necessary to prevent the retractile form of restenosis, their presence in the artery can lead to biological problems including vasospasm, compliance mismatch, and even occlusion. Moreover, there are inherent, significant risks from having a metal stent permanently implanted in the artery, including erosion of the vessel wall. The stents may also migrate on occasion from their initial insertion location raising the potential for stent-induced blockage. Metal stents, especially if migration occurs, cause irritation to the surrounding tissues in a lumen. Also, since metals are typically much harder and stiffer than the surrounding tissues in a lumen, this may result in an anatomical or physiological compliance mismatch, thereby damaging tissue or eliciting unwanted biologic responses. In addition, the constant exposure of the stent to the blood can lead to thrombus formation within the blood vessel. Stents also allow the cellular proliferation associated with the injured arterial wall to migrate through the stent mesh, where the cells continue to proliferate and eventually lead to the narrowing of the vessel. Further, metal stents typically have some degree of negative recoil. Finally, metallic stents actually prevent or inhibit the natural vascular remodeling that can occur in the organism by rigidly tethering the vessel to a fixed, maximum diameter.
Because of the problems of using a metallic stent, others have recently explored use of bioabsorbable and biodegradable materials stents. The conventional bioabsorbable or bioresorbable materials from which such stents are made are selected to absorb or degrade over time. This degradation enables subsequent interventional procedures such as re-stenting or arterial surgery to be performed. It is also known that some bioabsorbable and biodegradable materials tend to have excellent biocompatibility characteristics, especially in comparison to most conventionally used biocompatible metals. Another advantage of bioabsorbable and biodegradable stents is that the mechanical properties can be designed to substantially eliminate or reduce the stiffness and hardness that is often associated with metal stents. This is beneficial because the metal stent stiffness and hardness can contribute to the propensity of a stent to damage a vessel or lumen. Examples of novel biodegradable stents include those found in U.S. Pat. No. 5,957,975, which is incorporated by reference in its entirety.
Under the previously employed procedure(s), the metallic and biodegradable stents would be rapidly expanded to balloon nominal diameter of (coronary balloon) from typically 2.0 mm to 5 mm; (vascular peripheral balloon (PTA)) from typically 3 mm to more than 20 mm depending on balloon diameter. From this initial expansion, the stent would often then modulate its diameter. For example, for a 3 mm balloon, the biodegradable stent—educated at a diameter of 3.2—would be expanded to 3 mm and when the balloon was removed, then gradually expand over hours and/or days to 3.2 mm. Rapid expansion, which is typical in the prior art of implanting stents, however, can adversely affect the mechanical properties of polymer and biodegradable stents.
Still others have contemplated deployment by heating polymer and biodegradable stents; however, again, a quick heating process can damage the mechanical properties of the stent and in the case of polymeric stents educated to specific diameter, heating can erase the pre establish education of a preprogrammed desired final diameter . It is desirable to avoid the time that a stent is exposed to adverse temperature conditions (i.e., greater than body temperature −37 degrees C.), thereby enabling greater memory retention of the polymers diameter.
The mechanical property damage that occurs during current stent deployment may contribute to known polymer and biodegradable stent problems. For example, testing in animals has shown that polymer and biodegradable stents still suffer from multiple complications, including breaking of stent struts, complete longitudinal severing of the stent resulting in complete loss of mechanical integrity and collapse of the stent, relaxation-related negative recoil, lack of sufficient radial strength, difficulty in deployment, and distal migration of the entire stent or portions thereof. These failures may lead to thrombosis and occlusion of the vessel being stented with dire consequences for the patient.
Accordingly, it is desirable to find novel stent deployment methods that minimize the potential damage to the stent. As such, the inventors have found a novel method to deploy the stent by use of various stepwise procedures of increasing the pressure/diameter over time to slowly increase the stent diameter and allowing for a period between the stepped , increases in pressure/diameter for the stent to acclimatize to its current diameter and stent wall stresses and strains.